(i) Field of the Invention
The present invention relates to a polishing method and a polishing device, particularly to a polishing method and a polishing device which can preferably be used for polishing an inner peripheral end surface or another surface of a glass substrate for a magnetic recording medium, and the like.
(ii) Description of the Related Art
An aluminum substrate has been broadly used as a substrate for a magnetic disc or another magnetic recording medium, but accompanying miniaturization/thinning of the magnetic disc and densification of recording, the aluminum substrate is increasingly replaced with a glass substrate which is superior to the aluminum substrate in surface flatness and substrate strength.
In general, a glass substrate chemically reinforced to raise the substrate strength, or a crystallized glass substrate whose strength is raised by crystallization is used as the glass substrate for the magnetic recording medium.
Moreover, for a magnetic head, high-density recording brings about a transition from a thin-film head to a magnetic resisting head (a magnetro-resistive head) (MR head) and a giant magnetic resisting head (a giant magnetro-resistive head) (GMR head). Therefore, it is expected that regeneration of the magnetic recording medium using the glass substrate with the magnetic resisting head becomes a mainstream from now on.
In this manner, various improvements are added to the magnetic disc for high-density recording, and with the progress of the magnetic disc, new problems in the glass substrate for the magnetic recording medium arise one after the other. One of them is high-level cleaning of a glass substrate surface. Specifically, foreign particles sticking to the glass substrate surface may cause defects of a thin film formed on the glass substrate surface, or form convex portions on a thin film surface. An adequate glide height cannot be obtained.
Moreover, in the regeneration of the magnetic recording medium using the glass substrate with the magnetic resisting head, if the flying height of the head is lowered to enhance recording density, regeneration error may occur, or regeneration cannot be performed. Such disadvantageous phenomenon is caused by thermal asperity resulting from the convex portions formed on the magnetic disc surface by the particles on the glass substrate. In this case, heat is generated on the magnetic resisting head (the magnetro-resistive head), the resistance of the head is fluctuated, and electromagnetic conversion is adversely affected.
The cause of the foreign particles on the surface of the glass substrate for the magnetic recording medium lies in that a glass substrate end surface having no smooth state grinds against a wall face of a resin case. Resin or glass particles generated by the grinding, and other particles caught by inner and outer peripheral end surfaces of the glass substrate stick to the surface. Inventors et al. have found that especially the inner peripheral end surface of the glass substrate is coarser than the outer peripheral end surface, more easily catches particles, and obstructs the high-level cleaning of the glass substrate surface.
Additionally, proposed is a technique of removing cracks generated on an end surface portion of a glass substrate by chemical etching to enhance a substrate strength (Japanese Patent Application Laid-open No. 230621/1995). In this case, the depth of the crack is decreased, but the crack is etched, enlarged and indented to easily catch particles, which further obstructs the high-level cleaning of the glass substrate surface. Moreover, in the chemical etching, it is difficult to control the surface precision of the end surface portion at a high level. Furthermore, it is difficult to completely remove cracks, and only an insufficient deflection strength is provided.
The present invention has been accomplished under the circumstances above, and a first object thereof is to provide a polishing method, a polishing device, and the like in which an end surface of a glass substrate or the like can efficiently be smoothed at low cost and high level, especially, an inner peripheral end surface of the glass substrate or the like difficult to be polished can efficiently be smoothed at low cost and high level, so that cleaning of a substrate surface can be achieved to a high level.
A second object is to provide a glass substrate for a magnetic recording medium in which cleaning of a glass substrate surface is achieved to a high level.
A third object is to provide a magnetic recording medium in which troubles caused by foreign particles on a substrate surface are minimized.
According to the first aspect of the invention, there is provided a polishing method in which an inner peripheral end surface and/or an outer peripheral end surface of a circular disc having a circular hole in a center portion is polished using an abrasive liquid containing free abrasive grains.
According to the second aspect of the invention, there is provided a polishing method in which a glass substrate in the shape of a circular disc having a circular hole in a center portion is immersed in an abrasive liquid containing free abrasive grains, and an inner peripheral end surface and/or an outer peripheral end surface of the glass substrate is polished using the abrasive liquid containing the free abrasive grains.
According to the third aspect of the invention, there is provided a polishing method in which a glass substrate in the shape of a circular disc having a circular hole in a center portion is immersed in an abrasive liquid containing the free abrasive grains, and an inner peripheral end surface and/or an outer peripheral end surface of the glass substrate is polished using the abrasive liquid containing free abrasive grains by rotating a polishing brush or a polishing pad in contact with the glass substrate.
In the polishing method, the viscosity of the abrasive liquid containing the free abrasive grains is preferably 1.5 to 25 cps.
Furthermore, there is disclosed a method of manufacturing a glass substrate for a magnetic recording medium which comprises a step of polishing an inner peripheral end surface and/or an outer peripheral end surface of the glass substrate in the polishing method described above.
According to the fourth aspect of the invention, there is provided a glass substrate for a magnetic recording medium in which surface roughness Ra of a chamfered portion and/or a side-wall portion of an inner peripheral end surface and/or an outer peripheral end surface is 0.001 to 0.04 xcexcm.
The magnetic recording medium can be prepared by forming at least a magnetic layer on the aforementioned glass substrate.
The magnetic recording medium can be used as a magnetic recording medium for a magnetic resisting head (a magnetro-resistive head) (MR head) or a giant magnetic resisting head (a giant magnetro-resistive head) (GMR head).
As the magnetic layer, a magnetic layer including Co and Pt can be used.
According to the fifth aspect of the invention, there is provided a polishing device which comprises an abrasive liquid container in which an abrasive liquid is contained, a rotary support disposed on a bottom of the abrasive liquid container, a substrate case detachably mounted on the rotary support for holding a large number of circular discs each having a circular hole in a center portion, and a rotary brush inserted in the circular holes of the circular discs.
Additionally, each of the inner and outer peripheral end surfaces in the present invention includes a chamfered portion and a side-wall portion.
According to the first aspect, by polishing the inner and outer peripheral end surfaces of the glass substrate or the like using the abrasive liquid containing the free abrasive grains, the surfaces can be smoothed more efficiently at lower cost and higher level, as compared with a polishing method in which a diamond abrasive wheel (fixed abrasive grain) or a chemical etching is used. Especially, the inner peripheral end surface of the glass substrate or the like difficult to be polished with high precision can efficiently be smoothed at low cost and high level. When the diamond abrasive wheel is used, only raised portions (apexes of protrusions) of the polished surface are scraped off, and poor smoothness is provided. In the chemical etching, cracks are etched, enlarged and indented to easily catch particles, which obstructs the high-level cleaning of the glass substrate surface. Moreover, the glass substrate end surface provides poor smoothness, and foreign particles are generated by frictional grinding or the like. Furthermore, since it is difficult to completely remove the cracks, inferior deflection strength is provided.
According to the second aspect, by immersing and polishing in the abrasive liquid containing the free abrasive grains, a sufficient amount of the abrasive liquid is allowed to exist on the inner peripheral end surface and/or the outer peripheral end surface. Therefore, polishing insufficiency or defect because of liquid shortage can be avoided.
According to the third aspect, by combining the immersion into the abrasive liquid containing the free abrasive grains with the brush polishing or the like, especially the inner peripheral end surface of the glass substrate or another surface difficult to be polished with high precision can efficiently be smoothed at low cost and high level. Moreover, not only the chamfered portions but also the side-wall portions of the inner and outer peripheral end surfaces of the glass substrate can also be smoothed efficiently at low cost and high level. If both the chamfered portion and the side-wall portion are smooth, the effect of the present invention is further enlarged.
Moreover, in a case where the inner peripheral end surface defining a small-diameter hole is polished with the rotary brush, in a mode of continuously supplying the abrasive liquid to the rotary brush, the abrasive liquid is scattered by the rotary brush rotating at a high speed, and the periphery of the rotary brush is brought in a vacuum state to reject the abrasive liquid. In this manner, the abrasive liquid is insufficiently applied to the polished surface. However, by the immersion into the abrasive liquid containing the free abrasive grains, polishing insufficiency or defect is prevented from occurring because of liquid shortage. Moreover, even when a rotary brush having elastic bristles is used, the elasticity of the bristles immersed in the abrasive liquid is moderated by the viscosity resistance or another property of the abrasive liquid. Since the bristles are prevented from colliding against the polished surface unnecessarily strong, a possibility of scratching or damaging the polished surface can remarkably be reduced. Furthermore, for example, by arranging the bristles helically on a rotation shaft, the fluidity of the abrasive liquid is prompted. Since a fresh abrasive liquid can constantly be circulated/supplied to the polished surface, polishing efficiency, reproducibility and precision can be enhanced.
By setting the viscosity of the abrasive liquid containing the free abrasive grains in a range of 1.5 to 25 cps (20xc2x0 C.), polishing efficiency, reproducibility and polishing precision can be enhanced, while the possibility of scratching or damaging the polished surface can remarkably be reduced. In this respect, the viscosity of the abrasive liquid containing the free abrasive grains is more preferably in a range of 1.8 to 5 cps (20xc2x0 C.). By employing the above-mentioned polishing method, the cleaning of the glass substrate surface can be achieved to a high level, and the glass substrate for the magnetic recording medium superior in deflection strength can be manufactured.
According to the fourth aspect, by defining the surface roughness of the chamfered portion and/or the side-wall portion of the inner peripheral end surface and/or the outer peripheral end surface in the glass substrate for the magnetic recording medium, the cleaning of the glass substrate surface can be achieved to a high level, while the glass substrate for the magnetic recording medium superior in deflection strength can surely be obtained. In this respect, the surface roughness of the chamfered portion and/or the side-wall portion of the inner peripheral end surface and/or the outer peripheral end surface in the glass substrate for the magnetic recording medium is more preferably set to Ra of 0.001 to 0.03 xcexcm and Rmax of 0.5 xcexcm or less, where Ra is representative of center-line mean roughness, where Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point, (defined in Japanese Industrial Standard JIS B 0601).
As aforementioned, since the surface of the glass substrate for the magnetic recording medium is cleaned to a high level and the deflection strength is enhanced, in the magnetic recording medium, foreign particles attributed to the end surface fail to stick to the glass substrate surface. No defect arises on the thin film formed on the glass substrate surface, and the glide height can be lowered.
In the present invention, even if the flying height of the head is lowered, the regeneration error attributed to thermal asperity or the impossibility of regeneration is prevented. Therefore, the magnetic recording medium for the magnetic resisting head (the magnetro-resistive head) (MR head) or the giant magnetic resisting head (the magnetro-resistive head) (GMR head) can be obtained.
Furthermore, the magnetic recording medium superior in magnetic characteristics can be obtained.
According to the fifth aspect, by the rotary motion of the rotary support and the rotary brush, the inner peripheral end surface or the like can be polished remarkably efficiently. A high-precision, high-efficiency polishing can be performed without damaging the polished surface. Moreover, the rotary motion of the rotary brush allows the abrasive liquid to circulate in a tank, and prevents the abrasive from being deposited. Additionally, in addition to the rotary brush, means for circulating the abrasive liquid may further be provided.